Coatings are effective for improving the performance of various materials, such as for achieving better wear resistance and corrosion resistance. Common applications where a coating is applied to a substrate to improve wear resistance of the substrate material include cutting tool inserts for the cutting of hard materials, such as steel. Common substrate materials for cutting tools may include, for example, hard metals of different particle sizes with a varied percentage of cobalt or nickel as a binder material.
Boron nitride (BN) is a well-known material that, due to its thermal and chemical stability, is widely used in the fabrication of parts for high-temperature applications. It exists amorphously and in several crystalline forms. Cubic boron nitride (cBN) is a crystalline form that, due to its very high hardness value and relatively low cost, is widely used in various cutting applications as an abrasive. Since it is relatively insoluble in iron, nickel, and related alloys at high temperature, it is widely used in place of diamond for the cutting and machining of steel.
The art also includes various methods of coating parts—such as inserts for cutting tools and machines—with one or more layers of cBN. The process used has been the application of cBN in pure, non-composite phase. This application of cBN layer by vapor deposition is made with a coating thickness limited to less than about 3 microns. Attempts to form thicker layers of cBN have resulted in poor performance, since the high stress at the coating-substrate interface leads to coating delamination, thereby reducing the life of a cutting tool or other coated material. It would be highly desirable to develop a method of applying a thicker cBN coating that would not delaminate, due to the improved toughness, thermal stability, and wear resistance that would result from a thicker cBN layer.
The inventors hereof have previously developed a method of forming a cBN composite layer on a substrate that has a greater thickness, up to about 20-30 microns, which is a range desirable for a number of applications, particularly turning applications. This process, however, has limitations with respect to its flexibility in controlling the density of the cBN composite layer, and with respect to increasing the thickness of the layer beyond this range. It would be desirable to develop an even thicker layer of cBN for certain applications, such as coatings used for certain parts used in deep oil and gas drilling, where a coating thickness of greater than a few hundred microns is typically desired. In addition, there have also been high-temperature, high-pressure (HT-HP) processes applied to the development of polycrystalline cubic boron nitride (p-cBN) tools. This approach is limited, however, to production of in-bulk materials, that is, materials with thicknesses in the millimeter range in the form of discs or wafers. These materials must be diced to certain dimensions and shapes in order to be brazed to other bodies in order to be used as a coating material. The brazing process requires that the thickness of the materials be typically greater than 1.0 mm. Finally, the prior art does not include means of producing cBN particles with a gradient of particle sizes in a cBN composite coating, which the inventors hereof believe may be desirable for certain applications, particularly where balanced toughness and hardness are desirable.
CVI is a known process for the deposition of one or more materials within an existing body of porous material matrix. It is a variation of chemical vapor deposition (CVD), which uses a similar process for the deposition of a material onto a surface. CVI has been used, for example, in the deposition of materials onto and into a material that is composed of porous fibers. To the inventors' knowledge, however, CVI has not been used in a process for forming a composite layer comprising cBN particles wherein a cBN layer is infiltrated with another material to form a composite.